This is a fine card. I run Valkyrie in almost all of my aggression decks, largely because there's a limited pool of options, and because she's okay.

Numerically, I'll break down her values:

• If we assume that the baseline for damage dealing in Marvel Champions is 2 damage for every 1 resource spent;
• Valkyrie costs 4~ resources (if you include herself), thus, we should expect at least 8 damage.
• With an attack value of 2 and a health value of 3, Valkyrie can attack 3 times for a total damage of 6.
• Her response effect guarantees an additional 2 damage with the potential 3 damage. This brings her total guaranteed damage to be 8, and possibly 9.

She's just okay. A lot of Hero-specific cards go above the 1 for 2 rule, but aspect cards generally don't. This card could be read as Pay 4; deal 8 damage (9 if you spent an energy) etc. but because it's an Ally, there are other benefits. Namely, if needed, she can Thwart - although poorly, and of course, body blocking with her final hit point.

Recursion effects like Rapid Response are also just "okay" with her. Valkyrie turns Rapid Response into a 3 cost, deal 6 (or 7) damage total. Which is better than cards like Uppercut

Synergistic effects like Steve Rogers or Avengers Assemble! are nice; but they aren't specifically intended to combo well with this card. If a hero/card releases that allows you to trigger an ally's "enter play" effects regularly (like on attack or something), that could be pretty good. Alternatively, a Hero/upgrade that causes all damage dealing abilities to deal 1 additional damage would incentivize including this in your deck.

All-in-all, I like Valkyrie. Her ability allows you to poke out Tough or low health minions and she's a good basic damage dealing ally for Aggression to have. Depending on how Marvel Champions evolves, I could see Valkyrie being played less and less.